PROJECT SUMMARY Pneumonia is the leading cause of death among patients with acute myeloid leukemia (AML). Despite constant exposure of an immense surface area of delicate tissue to the external environment, the lungs' intrinsic defenses clear most aspirated and inhaled pathogens before infections are established. These same mucosal defenses can be therapeutically stimulated using a novel inhalational therapy comprised of a non-intuitive Toll- like receptor (TLR) agonist combination. This inducible resistance is associated with rapid intrapulmonary pathogen killing and prevents death in mice from otherwise lethal pneumonias caused by common AML- associated pathogens, even in the setting of severe chemotherapy-induced immunocompromise. The discovery that lung epithelial cells are principle effectors of the inducible response makes this approach particularly appealing for use in neutropenic AML patients. This application proposes to dissect the molecular mechanisms underlying this remarkable phenomenon to aid clinical translation of this technology for use in AML patients and to advance understanding of novel host-pathogen interactions. Aim 1 will identify the lung epithelial cell populations required for inducible resistance against AML-associated pneumonia. Contributions will be established by comparing inducible killing of AML pathogens by primary mouse and human epithelial cells and by functional testing of mice cell-selectively deficient in TLR signaling. Aim 2 will determine whether inducible resistance is impaired by leukemia cells or by treatment with standard cytotoxic or hypomethylating AML regimens. This will be assessed in vivo and in vitro based on effects on survival, pathogen killing, epithelial vitality, cellular activation, antimicrobial effectors, and circulating leukocytes Aim 3 will dissect the molecular mechanisms of inducible resistance to determine whether protection can persist despite co-administration of modern targeted molecular AML treatments, and to facilitate discovery of novel epithelial stimuli. Mouse genetic manipulation and in vitro models will identify required signaling and effector molecules and are expected to provide insight into the unexplained TLR synergy observed. The proposed studies are expected to identify critical cells, signaling pathways, and effector molecules of inducible resistance, promote discovery of more efficacious inducers of resistance, explore unanticipated TLR interactions, assess interactions of inducible resistance with AML and its treatments, identify AML populations most likely to benefit from the treatment, and facilitate the rapid translation of this technology into the clinic, so that AML patients can be protected from lethal pneumonia during periods of peak vulnerability.